This invention relates to electrolytic production of aluminum and more particularly it relates to a method of maintaining the molten salt electrolyte concentration constant in a low temperature electrolytic cell used for the production of aluminum from alumina dissolved in the molten electrolyte.
The use of low temperature (less than about 900° C.) electrolytic cells for producing aluminum from alumina have great appeal because they are less corrosive to cermet or metal anodes and other materials comprising the cell. The Hall-Heroult process, by comparison, operates at temperatures of about 950° C. This results in higher alumina solubility but also results in greater corrosion problems. Also, in the Hall-Heroult process, carbon anodes are consumed during the process and must be replaced on a regular basis. In the low temperature cells, non-consumable anodes are used and such anodes evolve oxygen instead of carbon dioxide which is produced by the carbon anodes.
The Hall-Heroult process has another disadvantage. That is, in the Hall-Heroult process, the cell operates with a solidified crust or layer that covers the molten electrolyte and thus for the periodic additions of alumina to the cell, the crust must be broken in order to add alumina or make alumina dumps to the molten electrolyte. This has the problem that large quantities of emissions, e.g., fluorides, are lost from the cell and usually are captured along with emissions from other cells. However, this has the problem that each cell operates differently, giving off different amounts of fumes. Thus, addition of make-up electrolyte based on an average is not satisfactory because the average can be too much for one cell and not enough for another, requiring frequent analysis of the electrolyte as well as frequent addition of significant mounts of electrolyte to maintain the desired molten electrolyte concentration.
Different processes have been suggested for operating electrolytic cells for the production of aluminum or feeding alumina to such cells.
For example, U.S. Pat. No. 5,779,875 discloses a method for feeding loose material such as alumina into an electrolytic cell, the method includes the following step of forming at least one material input zone on the surface of an electrolyte, disposing a working tool in the input zone to push the material into the melt of the electrolyte, imparting mechanical oscillations and translatory motions to the working tool in the direction towards the electrolyte and back therefrom with the length of said motions being within the range of values from about 10.0 to about 120.0 sec. The method further includes transporting material into the input zone and forming some layer in the material input zone, and after accumulation of a sufficient amount of the material, the latter enters into contact with the working tool. The aforesaid steps increase the capacity of the material input zone and reduce power expenditures on the input of material into the electrolyte.
Also, U.S. Pat. Nos. 5,415,742 and 5,279,715 disclose a process for electrowinning metal in a low temperature melt. The process utilizes an inert anode for the production of metal such as aluminum using low surface area anodes at high current densities.
U.S. Pat. No. 5,089,093 discloses a process for controlling an aluminum smelting cell comprising monitoring the cell voltage and current, alumina dumps, additions, operations and anode to cathode distance movements, continuously calculating the cell resistance and the bath resistivity from said monitored cell voltage and current, monitoring the existence of low frequency and high frequency noise in the voltage of the cell, continuously calculating the time rate of change of resistance of the cell, suspending calculation for a predetermined time when an alumina dump is made.
U.S. Pat. No. 4,766,552 discloses a method for controlling alumina feed to reduction cells for the production of aluminum. The method employs an adaptive control with parameter estimation (3) and controller calculation (2) based upon the separation theorem. As a process model there is used a linear model having two inputs and one output. One input (u1) is in the form of alumina feeding minus assumed alumina consumption. Another input (u2) is in the form of movements of the reduction cell anode. The output (y) is in the form of the change in electric resistance across the reduction cell concerned. The model is of the first order in u1 and u2 whereas it is of the order zero in y. An estimated parameter (b1) represents the slope of the curve for resistance as a function of alumina concentration in the electrolytic bath, and the controller (2) controls the addition of alumina to the electrolytic bath in response to the value of b1.
U.S. Pat. No. 4,101,393 discloses a method for the controlled cleaning of aluminum chloride contaminated filtering means used in a system for recovery of gaseous effluents formed in the production of aluminum from aluminum chloride. The method includes transferring filtering units from the system to a cleaning vessel, placing them inside the vessel and sealing the vessel from the environment. Water is flowed into the lower portions of the vessel to immerse the units to cause the aluminum chloride to react with the water, giving off gaseous and liquid products of reaction. Gaseous materials are exhausted from the upper portion of the vessel to a fugitive gas system and the liquid products are discharged from a separate exhaust means. Filtering materials are then stripped from the units to be disposed of without polluting the environment. An apparatus is also provided for carrying out the method of the present invention.
U.S. Pat. No. 4,176,019 discloses that in the scrubbing of gases containing sorbable contaminants, particularly the waste gases from reduction cells for electrolytic production of aluminium the waste gas is injected tangentially into the bottom of a cylindrical chamber, from which it is withdrawn through an axial outlet passage at the top end. A solid sorbent material is introduced into the chamber at one or more positions at the top end of the chamber in such a way that it enters the ascending gas stream in a peripheral zone of the chamber.
U.S. Pat. No. 4,431,491 discloses a process and apparatus for controlling the rate of introduction and the content of alumina to a tank for the production of aluminium by the electrolysis of dissolved alumina in a cryolite-base bath, the upper part of which forms a solidified crust, and wherein the alumina content is maintained within a narrow range, of between 1% and 3.5%, wherein the alumina is introduced directly into the molten cryolite bath by way of at least one opening which is kept open in the solidified crust and the rate at which the alumina is introduced is modulated relative to variations in the internal resistance of the tank during predetermined periods of time, with alternation of the cycles of introducing alumina at a slower rate and at a faster rate than the rate corresponding to normal consumption within the tank.
U.S. Pat. No. 4,814,050 discloses a method of estimating and controlling the concentration of alumina in the bath of a Hall cell. The method includes the use of an estimator that employs two sets of equations, namely, a time update algorithm that contains a dynamic model of the alumina mass balance of the cell and provides estimates of alumina concentration, and a measurement algorithm that uses a process feedback variable from the cell to modify the alumina estimate. In addition, the method includes the use of one or more tuning parameters, such as state noise variance and measurement noise variance. The measurement noise variance is modified by the process noise variance in a manner that increases measurement noise variance for high values of process noise and decreases measurement noise variance for low values of process noise. In addition, one or more of the parameters of the model are modified by the feed history of the cell.
U.S. Pat. No. 5,505,823 discloses a process for smelting aluminum from a mixture of a double salt potassium-aluminum sulfate 2KAl(SO4)2 and aluminum sulfate Al2(SO4)3 with potassium sulfate K2 SO4 having a weight ratio of 2KAl(SO4)2 to K2 SO4 in the range of 50/50 to 15/85. The mixture is heated to a eutectic temperature that makes it molten and electrolysis is used to precipitate out aluminum at the negative electrode and gases from SO4 ions at the positive electrode. A critical amount of a feed of 2KAl(SO4)2 is added to replace that which was consumed in the electrolysis and to maintain the weight ratio which provides for the low eutectic melting temperature.
U.S. Pat. No. 5,968,334 discloses a process for recovering at least one of CF4 and C2 F6 from a vent gas from an aluminum electrolysis cell.
U.S. Pat. No. 2,713,024 discloses a process which comprises maintaining the bath at crust-forming temperature conditions, continuously feeding alumina through the crust into the bath, and applying feed pressure through the alumina to the bath surface to thereby penetrate the crust.
U.S. Pat. No. 4,654,129 discloses a process for accurately maintaining a low alumina content of between 1 and 4.5% in a cell for the production of aluminum by electrolysis in the Hall-Heroult process. According to the invention, a control parameter P=−1/D(dR1/dt), is determined, wherein D is the variation in the alumina content of the electrolytic bath in % weight per hour, R1 is the internal resistance of the cell, and t is the time. A series of operations is then carried out in a repeated cycle, starting with the cell being fed alumina at a nominal rate which is substantially equal to the quantity consumed by electrolysis. At periodic intervals, an over-supply of alumina is added in order to enrich the bath, and the over-supply is continued for a preset time during which dR1 dt is negative. The feed rate is then reduced to less than the nominal feed rate, during which time dR1 dt passes through zero to become positive and the regulation parameter P, the value of which tends to rise, is measured often. The successive values of P are compared with a required preset value Po. As soon as P equals Po, the feed rate is returned to the nominal feed rate and a new cycle is recommenced.
U.S. Pat. No. 4,333,803 discloses a method and apparatus for maintaining a predetermined energy balance in a device, such as an aluminum reduction cell. The apparatus includes a relatively short and thin heat flow sensor having a first and second thermocouple located within opposite closed ends of a hollow thermally conductive body. Each thermocouple is composed of two wires of the same dissimilar metals. The sensor is secured by one closed end of the sensor body to an outside surface of the wall member to extend substantially perpendicular to the location on the wall without significantly affecting the heat flow from the wall surface being measured. A first wire of each thermocouple is of the same metal for electrically connecting the junctions of each thermocouple. The second wire of each thermocouple extends to a location intermediate the closed ends of the sensor body and is electrically connected to an instrument responsive to the electrical potential between the first and second thermocouples. A control means uses the electrical signal to determine the heat flow through the wall member as a function of the temperature difference between the thermocouples and to maintain a predetermined energy balance of the system by adjusting the amount of energy added to the system. The method and apparatus also control the frozen lateral ledge thickness of an aluminum reduction cell.
In spite of these disclosures, there is still a great need for a process that returns electrolytic cell emissions such as fluoride emissions to an electrolytic cell and preferably returns such emissions to the same cell from which they were vented in order to maintain the molten electrolyte at the desired concentration.